420 THE BLOOD, BY ALEXANDEK KOLLETT. 



In the tail of young tadpoles the newly formed vessels are 

 found to be filled with peculiar, short, compressed, fusiform 

 bodies, flattened on two of their sides, which present a very 

 light yellow tint, and contain numerous yolk granules, but 

 are otherwise homogeneous. 



In addition to these primary cells there appear, it would 

 seem, concomitantly with the progressive development of the 

 intestinal tract, a constantly increasing number of white cor- 

 puscles. The number of the cells filled with yolk granules, 

 on the other hand, gradually diminishes. We soon after meet 

 with the intermediate forms already described as existing in 

 adult animals, together with coloured blood corpuscles of the 

 form ordinarily present in the blood of the Frog. 



In Mammals there may be observed in the blood of the 

 embryo, at an early stage, nucleated coloured blood corpuscles 

 in process of fission. At a later period these forms are less 

 abundant, in accordance with the progressive development of 

 the embryo and of the spleen in particular (Kolliker), and 

 we meet with numerous white corpuscles in the blood of 

 the liver, which become metamorphosed into coloured nucle- 

 ated blood corpuscles. Up to a certain period of embryonic life 

 only nucleated red blood corpuscles are present in the blood 

 (Kolliker). The non-nucleated first appear at a later period, 

 their relative number then undergoing a constant increase. 

 According to Kolliker, non-nucleated corpuscles are not present 

 in the blood of foetal sheep measuring three and a half inches 

 in length; in those of nine inches long they are but seldom found, 

 whilst they constitute the majority in foetuses that are thirteen 

 inches in length. According to Robin,-)- in human embryoes 

 measuring thirty millimeters, about one half of the total num- 

 ber of blood corpuscles are destitute of nuclei ; a few nucleated 

 corpuscles are still discoverable in embryoes of the fourth 

 month, and even at still later periods. 



As has already been mentioned, the red blood corpuscles can 



* Kolliker, Zeitschrift fur rationelle Medicin, Band iv., p. 112; Gewe- 

 belehre. Leipzig, 1867, p. 637. E. H. Weber and Kolliker, Zeitschrift fur 

 rationelle Medicin, Band iv., p. 160. 



t Journal de la Physiologic. Paris, 1858, p. 288. 



